38 
ELEMENTS OF GEOLOGY. 
The above-mentioned three classes of rocks, the siliceous, 
argillaceous, and calcareous, pass continually into each oth¬ 
er, and rarely occur in a perfectly separate and pure form. 
Thus it is an exception to the general rule to meet with a 
limestone as pure as ordinary white chalk, or with clay as 
aluminous as that used in Cornwall for porcelain, or with 
sand so entirely composed of siliceous grains as the white 
sand of Alum Bay, in the Isle of Wight, employed in the 
manufacture of glass, or sandstone so pure as the grit of 
Fontainebleau, used for pavement in France. More com¬ 
monly we find sand and clay, or clay and marl, intermixed 
in the same mass. When the sand and clay are each in con¬ 
siderable quantity, the mixture is called loam. If there is 
much calcareous matter in clay it is called marl; but this 
term has unfortunately been used so vaguely, as often to be 
very ambiguous. It has been applied to substances in which 
there is no lime; as,to that red loam usually called red marl 
in certain parts of England. Agriculturists were in the 
habit of calling any soil a marl which, like true marl, fell to 
pieces readily on exposure to the air. Hence arose the con¬ 
fusion of using this name for soils which, consisting of loam, 
were easily worked by the plough, though devoid of lime. 
Marl slate bears the same relation to marl which shale 
bears to clay, being a calcareous shale. It is very abundant 
in some countries, as in the Swiss Alps. Argillaceous or 
marly limestone is also of common occurrence. 
There are few other kinds of rock which enter so largely 
into the composition of sedimentary strata as to make it 
necessary to dwell here on their characters. I may, how¬ 
ever, mention two others—magnesian limestone or dolomite, 
and gypsum. Magnesian limestone is composed of carbon¬ 
ate of lime and carbonate of magnesia; the proportion of 
the latter amounting in some cases to nearly one half. It 
effervesces much more slowly and feebly with acids than 
common limestone. In England this rock is generally of a 
yellowish color; but it varies greatly in mineralogical char¬ 
acter, passing from an earthy state to a white compact stone 
of great hardness. Dolomite,, so common in many parts of 
Germany and France, is also a variety of magnesian lime¬ 
stone, usually of a granular texture. 
Gypsum is a rock composed of sulphuric acid, lime, and 
water. It is usually a soft whitish-yellow rock, with a tex¬ 
ture resembling that of loaf-sugar, but sometimes it is entire¬ 
ly composed of lenticular crystals. It is insoluble in acids, 
and does not effervesce like chalk and dolomite, because it 
does not contain carbonic acid gas, or fixed air, the lime be- 
